1. Field of the Invention
The invention relates to a method of producing a displacement plating precursor and more particularly relates to a method of producing a displacement plating precursor. This method enables the production of core-shell particles in which the surface of a core particle formed of Pt or a Pt alloy is coated by a monoatomic layer of Cu, without using an external power source.
2. Description of Related Art
In general, a surface treatment, e.g., plating, physical vapor deposition (PVD), chemical vapor deposition (CVD), and so forth, is frequently carried out by covering the surface of a substrate with a different type of material in order to modify only the properties of the substrate surface while leaving the properties of the substrate intact. On the other hand, this type of surface treatment is also employed in order to lower the amount of use of an expensive material or improve its utilization rate or prevent a deterioration in its properties. For example, a catalyst (Pt-loaded carbon (Pt/C)) in which Pt or a Pt alloy having a particle diameter of several nanometers is supported on a carbon particle having a high specific surface area is generally used as an electrocatalyst for fuel cell applications. Since only the atoms residing in the vicinity of the surface of the Pt particle exercise a catalytic function, a problem with this PVC catalyst is the low utilization rate of the expensive Pt. As a consequence, an art has been proposed in which the interior of the particle is replaced by a different material and the use of Pt is limited to several atomic layers from the particle surface. Conventionally, the Pt particles grow larger and cell performance declines when Pt/C is used in a fuel cell environment. Due to this, an art has been proposed in which Pt particle enlargement is inhibited by forming a thin Au layer on the surface of the Pt particle.
Various proposals have already been made in relation to this surface treatment art. For example, the following method for producing an oxygen-reducing electrocatalyst has been disclosed in Published Japanese Translation of PCT application No. 2009-510705 (JP-A-2009-510705): (1) placing approximately 3 to 10 nm Pt nanoparticles on a suitable electrode and immersing this electrode in an approximately 50 CuSO4/0.10 M H2SO4 aqueous solution in a nitrogen atmosphere; (2) depositing a monoatomic layer of copper on the Pt nanoparticles by applying a suitable reducing potential to the electrode; (3) rinsing the electrode containing copper-coated Pt nanoparticles with purified water to remove the Cu ion present in the solution; (4) displacing the copper monoatomic layer with a gold monoatomic layer by immersing the electrode containing copper-coated Pt nanoparticles for 1 to 2 minutes in an approximately 1.0 mM aqueous HAuCl4 solution; and (5) rinsing the electrode again. It is stated in this publication that the Pt can be protected from oxidation and dissolution by coating the Pt nanoparticle surface with the Au monoatomic layer.
In the method of producing an oxygen-reducing electrocatalyst disclosed by J. Zhang et al., Science, 315 (2007) pp. 220-222; a Cu monoatomic layer is coated on a Pt surface by an underpotential deposition (URD) technique and the Cu monoatomic layer on the Pt is then galvanically displaced by Au. It is stated in this publication that (1) this method can provide an Au/Pt/C catalyst in which Au clusters are deposited on the Pt catalyst (Pt nanoparticles supported on carbon); and (2) when the Pt nanoparticles are modified by the Au clusters, there is almost no change in the activity and surface area of the Au-modified Pt even after the application of 30,000 0.6-to-1.1 V potential cycles under an oxidizing atmosphere.
In the method of producing a Pt monoatomic layer electrocatalyst disclosed by M. B. Vukmirovic et al., Electrochimica Acta, 52(2007) pp. 2257-2263, a Cu monoatomic layer is formed using a UPD technique on the surface of a Pd(111) single crystal or on the surface of a Pd nanoparticle and the Cu monoatomic layer is galvanically displaced by Pt. It is stated in this publication that the oxygen reduction reaction activity of the PtML/Pd/C is higher than that of Pt/C.
UPD is a phenomenon in which a metal ion in solution deposits onto a substrate metal different from the metal ion at a more noble potential (a potential at which deposition of the metal ion does not ordinarily occur) than the thermodynamic equilibrium potential for the metal ion. UPD is characterized by the ability to coat a substrate metal surface with a monoatomic layer of a different metal, but the metal combinations that support UPD are limited since it occurs when there is substantial interaction between the different metals. The deposition of a monoatomic metal layer by UPD is ordinarily carried out by immersing a working electrode (WE) to which the substrate metal is connected, a counterelectrode (CE), and a reference electrode (RE) in a solution that contains the metal ion and holding the WE at a prescribed potential using an external power source.
For example, when a Cu monoatomic layer is deposited on the surface of a Pt nanoparticle (or a Pt nanoparticle supported on a support such as carbon) using the UPD technique, the Pt nanoparticle must be coated and immobilized on the surface of the WE in order to hold the Pt nanoparticle at the prescribed potential using an external power source. However, this coating procedure has a high process cost and a large amount of time is required for the process. In addition, while being suitable for synthesis at the μg level, it is not well adapted for mass production.
Moreover, when the Pt nanoparticles immobilized on the WE are held at a prescribed potential using an external power source, an oxidation reaction occurs at the CE at the same time that the Cu ion reduction reaction occurs at the WE. For example, oxygen gas is produced at the CE when the metal ion-containing solution is an aqueous sulfuric acid solution. When the generated oxygen comes into contact with the Pt nanoparticles in this case, the potential of the Pt nanoparticles increases and detachment of the deposited Cu monoatomic layer can be a problem. In order, on the other hand, to prevent detachment of the Cu monoatomic layer, it is then necessary to design the CE so as to prevent the generated gas from influencing the WE, while maintaining ionic conduction to the WE. However, it is considered to be quite difficult to design a reaction vessel that solves these problems and at the same time is well adapted for mass production.